This invention relates to calendered laminates of spunbonded and melt-blown polypropylene webs which combine high opacity with low air and water permeability but good moisture vapor transmission, and good barrier properties against bacteria.
Spunbonded polyethylene sheet (prepared from flash-spun polyethylene plexifilamentary web) has been used for some years as an air flow resistant "housewrap", as well as in sterile packaging applications. The sheet is known for its strength, durability, opacity and ability to reduce air penetration, yet maintains a high moisture vapor transmission rate and, in addition, provides excellent barrier to bacteria. However, there exists a need for a housewrap product with higher tear resistance, especially in areas exposed to strong winds. At the same time, there is a need in sterile packaging for a higher melting material, capable of withstanding steam sterilization. One polyolefin material that has sufficiently high melting temperature and excellent mechanical properties is polypropylene.
Use of polypropylene housewrap sheets is known. Thus, U.S. Pat. No. 4,898,761 to Dunaway et al. (Reemay Inc.) discloses a barrier housewrap fabric impermeable to liquid and permeable to water vapor, which is made by laminating an impermeable polymer film to a polypropylene sheet available, i.a., from E.I. du Pont de Nemours and Company under the trademark TYPAR.RTM. and needling the resultant sheet with fine diameter needles to make the sheet porous. U.S. Pat. No. 4,684,568 to Lou (DuPont) describes a water vapor-permeable, liquid water-impenueable fabric made by applying a coating of polypropylene film to a TYPAR.RTM. sheet and then calendering. Although these products have adequate tear resistance and strength, they do not have a good balance of air penetration resistance and moisture-vapor transmission rate.
U.S. Pat. No. 4,766,029 to Brock et al. (Kimberley-Clark Corp.) discloses a housewrap which is a calendered, three-layer laminate of spunbonded/melt-blown/spunbonded webs. The melt-blown layer has two components, namely, polyethylene and polypropylene microfibers. During lamination, polyethylene fibers melt and the material flows, to close up the interstitial space and to bond the layers together. This creates a semi-permeable laminate. The requirement of two types of fibers in the melt-blown material is on obvious disadvantage.
U.S. Pat. No. 4,900,619 to Ostrowski et al. (James River Corp.) describes a translucent nonwoven composite of thermoplastic melt-blown and spunbonded webs laminated together by hot-calendering in a nip made by a smooth, heated steel roll and a resilient roll heated with an external, infrared source, each roll being at a temperature of about 116.degree.-160.degree. C. Typically, the thermoplastic material in each layer is polypropylene.
The term "translucent" is understood to mean "permitting the passage of light". The degree of translucency can be determined, e.g., by an opacity measurement (TAPPI test method T 425 om-86, "Opacity of Paper"). By that test, the commercial James River product has an opacity of less than 50%. It is possible that this low opacity results from laminating the sheets between two heated calender rolls, forming a film. Usually, translucency indicates the occurrence of permanent changes of certain physical properties of the fibers or of their distribution in the sheet, which could lead to lower mechanical integrity or tensile strength. Further, it is preferred, for aesthetic reasons, to have a housewrap that is opaque, rather than translucent. Even though the housewrap eventually is covered by a siding, a house under construction covered with a housewrap through which all the studs and joints can be seen appears unsightly.
International Application WO 87/05952 of Kimberley-Clark discloses impregnation of spunbonded filaments with a fluorocarbon prior to calendering. The product is a spunbonded/melt-blown/spunbonded nonwoven laminate for disposable garments. The purpose of calendering is to improve the resistance of the laminated garment's surface to "fuzzing and linting", while maintaining porosity, softness and drape. Without fluorocarbon impregnation, garments lose their porosity during calendering. The calender comprises a smooth steel roll heated to the melting temperature of the fibers in the layer in contact therewith (e.g., 167.degree. C. for polypropylene) and an unheated roll which can be made of such materials as plastic, cotton, or paper.
There is a need for a strong, opaque web with good water vapor penetration rate, low air permeability, no liquid water permeability, and good barrier properties against bacteria.